Methotrexate and Thiotepa in Treating Patients With Newly Diagnosed Primary CNS Lymphoma

DISEASE CHARACTERISTICS: * Histologically confirmed primary CNS lymphoma * Confirmed by 1 of the following: * Brain biopsy or resection * CSF cytology * Positive cytology or immunohistochemical diagnosis of monoclonality and measurable intracranial tumor * Vitreal biopsy * Measurable and contrast-enhancing disease on the postoperative, pretreatment MRI or CT scan * No radiographic evidence of ascites or pleural effusions PATIENT CHARACTERISTICS: Age * 18 and over Performance status * Karnofsky 60-100% Life expectancy * Not specified Hematopoietic * Absolute neutrophil count at least 1,500/mm\^3 * Platelet count at least 100,000/mm\^3 Hepatic * Bilirubin no greater than 2.0 mg/dL * SGOT no greater than 4 times upper limit of normal Renal * Creatinine no greater than 2 mg/dL * Creatinine clearance at least 50 mL/min Other * Mini mental score of at least 15 * HIV negative * Able to achieve hydration * No other malignancy within the past 5 years except curatively treated basal cell or squamous cell skin cancer or carcinoma in situ * No allergy to methotrexate * No serious infection * No medical illness that would preclude study participation * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy * No prior immunotherapy or biologic therapy for this disease Chemotherapy * No prior chemotherapy for this disease * No other concurrent chemotherapeutic agents Endocrine therapy * No prior hormonal therapy for this disease * Prior glucocorticoid therapy allowed Radiotherapy * No prior radiotherapy for this disease * No prior cranial irradiation Surgery * See Disease Characteristics Other * At least 1 week since prior salicylates, non-steroidal anti-inflammatory drugs, probenecid, folic acid, or sulfonamide medications * No other concurrent investigational agents